Immersion tin plating and composition therefore



United States Patent C INIMERSION TIN PLATIN G AND COIVIPOSITION THEREFORE Michael A. Streicher, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Dec. 24, 1958, Ser. No. 782,632 9 Claims. (Cl. 117-37) This invention relates to the immersion deposition of tin and is more particularly directed to processes and compositions useful for applying tin coatings to selected areas of steel sheets so that such areas can be soldered.

I have found that tin can be deposited on steel articles by immersion in an aqueous solution of stannous sulfate, sodium bisulfate, and a polyalkylene oxide. Sheet steel such as that used for making tin cans can be prepared for soldering by mechanically toughening selected areas which it will later be desired to solder and then immersing such sheet into the solutions of the invention to obtain a tin coating on the roughened areas.

According to the invention, a steel sheet is first roughened mechanically. This mechanical treatment is required in order to obtain deposition of tin and only those areas which have been roughened will receive tin using the baths which will hereafter be described.

The mechanical roughening can be effected in any convenient way as by sanding, scraping, or grinding with abrasive materials. Much preferred is grinding with an abrasive which will leave the surface roughened without excessive removal of metal. Preferably the surface can be roughened as with an SO-grit sanding belt or its equivalent. Obviously considerable variation is permissible in the grit depending upon the surface to be treated and the speed of travel of a-sanding belt or grinding wheel.

The surfaces to be coated with tin according to the invention should be clean in accordance with customary practices. They can be cleaned prior to roughening or after roughening. The surfaces can be cleaned using any conventional degreasing cleaning system followed by a conventional alkali cleaning. It is to be noted that if the surface is cleaned prior to mechanical roughening, then after such roughening any dust or grit remaining on the sheet should be removed by brushing or blowing.

If the immersion deposition of tin is to be effected at a rapid rate, which is a very desirable practice in the coating of sheet to be used in making tin cans, then the sheet ought to be preheated. This can be done by passing it through a hot water bath, by ending the cleaning cycle with a hot rinse, or of course the sheet can simply be passed through an area appropriately heated by dry heat.

The aqueous solutions employed according to the invention contain sodium bisulfate and stannous sulfate.

While the proportions of the sodium bisulfate and stannous sulfate can be considerably varied it has been found for best results the solution can contain:

. G. /l Sodium bisulfate 100-250 Stannous sulfate 10- 40 Water, 1 liter.

The amount of water can be considerably varied but the proportions shown will be found very satisfactory.

If the proportion of sodium bisulfate is lowered or raised much below the figure shown, the rate of deposiice tion will fall off so that the process becomes undesirably slow. About the same consequences result from raising or lowering the stannous sulfate content.

In addition to the components shown the composition must contain an Organic addition agent. The addition agent should be of the type shown in the Hoffman U.S. Patent 2,457,152. Any of the substituted or unsubstituted polyalkylene oxide addition agents there described can be employed, the amount in the above composition being 30-100 g./l. If more or less than the amount shown is used the effect on deposition rate will not be as great as may be desired.

In addition to the polyalkylene oxide addition agents described in the Hofiman patent and in the references therein cited there can be used any of a number of more recently developed substituted or unsubstituuted polyalkylene oxides of the same type. These will include an unsubstituted polyethylene oxide (now often called polyethylene glycol) having a molecular weight ranging up from about 350 and will also include unsubstituted polyethylene oxides having molecular weights as high as 20,000.

Broadly there may be used any of the agents of the types described in the said Hoffman patent, but it is more specifically preferred to use an unsubstituted polyethylene oxide having a molecular weight between 350 and 20,000. It is still more preferred to use an unsubstituted polyethylene oxide having a molecular weight between 6000 and 7500. It is most preferred to use an unsubstituted polyethylene oxide having a molecular weight of about 6000. I

A detailed description of the substituted and unsubstituted polyalkylene oxides of the Hoffman patent will be found therein and need not be repeated in detail. The disclosure of the said patent is incorporated here by reference.

Preferred compositions of the invention are dry, or pasty, products containing an appropriate mixture of sodium bisulfate with stannous sulfate and a polyalkylene oxide addition agent.

A preferred dry composition to be sold and to be used after dissolving in water is as follows:

Parts by weight -Stannous sulfate 1- 4 Sodium bisulfate 10-25 Unsubstituted polyethylene oxide having a molecular weight of about 6000 3-10 Stannous sulfate 10- 40 Unsubstituted polyethylene oxide having a molecular weight of about 6000 30-100 In processes of the invention a strip of steel will ordinarily be roughened on the surfaces to be plated and a solution as described will be applied. This can be done by immersing the surface or the whole sheet in the solution to effect deposition without the application of an external electric current. Whichever is done, the tin will deposit only on the roughened areas so that the remainder of the sheet will not be coated. The solution can be applied in any other similar fashion as by flowing the solution on the strip, by spraying, and so forth.

"The treating solutions of the invention should be used at elevated temperatures and temperatures that are near cordance with standard practices.

It will be seen that by coating only the edges or other selected areas of the strip-it is possible to form cans which have tin only at the soldered joints. There need be no waste of tin and there need be no tin present where it would not be wanted.

' In order that the invention may be better understood reference should be had to the following illustrative examples. 1

' Example 1 Steel strip to be made into cans was first cleaned in the following sequence. It was first roughened on the areas to beplatedby grinding with. an abrasive belt. 'In this particular case, the roughening was on opposite sides .at' thetwo edges, these sides beingthose which would be in contact in soldering of the side-seams of the can. The abrasive belt carried an 80-grit abrasive. After roughening the edges, the sheet was degreas'ed in an 'organicsolvent in conventional manner and dried in warm air.

7 The strip was then pre-heated in boiling water.

The strip was then immersed in a solution with the following components in the proportions shown:

'Water, suflicient to make 1 liter total. r V Stannous sulfate 10 Sodium bisulfate 100 Polyethylene oxide 520-600 molecular weight 27 The solution was at the boiling point. The strip was in contact with the bath for minutes and received a satisfactory coat of tin on the roughened surfaces, the deposit amounting to 76.7 mg. tin per sq. dm. There was no tin deposition on other areas.

The strip was then rinsed in weak sodium hydroxide solution (0.01 N). The strip was thereafter rinsed in warm water and dried.

The strip thereafter can be soldered readily to form a tin can. e

Example 2 A procedure was followed as in Example 1 except that instead of polyethylene oxide above described there was used a polyethylene oxide of'molecular weight 6000 the immersion time can be increased or lowered. Thus at the following immersion time's theresults obtained are Water, sufiicient to make 1 liter total.

81011 tin coating consisting essentially of:

'Unsubstituted polyethylene oxide having a molecushown, but with much lesser amounts of deposition with each solution.

Example 3 Parts by weight Stannous sulfate 1- 4 Sodium bisulfate 10-25 lar weight of about 6000 3-10 2. An aqueous bath for immersion tin coating consisting essentially of the following in the proportions tabulated: V

Sodium bisulfate"..' 1 100-250 Stannous sulfate '10- 40 Unsubstituted polyethylene oxide having a molecular' weight-of about 6000 3. A compositionto be dissolved in water for immer- Parts by weight Stannous sulfate Sodium bisulfate 10-25 Polyalkylene oxide of molecular weight from 350 to 20,000 I 3-10 4. An aqueous bath for immersion tin coating consisting consisting essentially of the following in the proportions tabulated:

' G./l. Sodium bisulfate 100-250 Stannous sulfate '10-40 Polyalkylene oxide of molecular weight from 350 to 20,000

30-100 Water,sufficient to make 1 liter total.

:5. A composition to be dissolved in water for immersion tin coating consisting essentially of;

Stannous sulfate 1-4 Sodium bisulfate 10-25 Unsubstituted polyethylene oxide with a molecular weight between 350 and 9000 3-10 6. An aqueous. bath for immersion tin coating consisting essentially of the following in the. proportions tabulated:

V V y p G./l. Sodium bisulfate 100-250 Stannous sulfate 10-40 'Unsubstituted polyethylene oxide with a molecular weight between 350 and 9000 30-100 as shown.

' Immersion time- Mg. Sn/sq. dm.

5 sec. 10.5 15 sec. 20.4

30 sec. I 34.8 A, 1 min; 46.7 2 min. d 189.7 7 5 min. 182.0 In the above examples other stannous compounds such as the hydroxy acetate, oxalate and sulfamate can be used instead of the sulfate in an amount to provide an equivalent quantity of tin,-all other conditions remaining as Water, sufficient to make 1 liter total.

7. In a process for depositing men a selected area ,of a steel sheet the steps comprising mechanically roughening such area and applying thereto an aqueous. bath consisting essentially of the following proportions of:

' G./l. Stannous sulfate i 10-40 Sodium bisulfate -250 Unsubstituted polyethylene oxide having a molecular weight of about 6000 30-100 Water, suflicient to make 1 liter total.

8. In a process for depositing tin on a selected area of a steel sheet the steps comprising mechanically roughening such area and applying thereto an aqueous bath consisting essentially of the following proportions of:

Water, suflicient to make 1 liter total.

9. In a process for depositing tin on a selected area of a steel sheet the steps comprising mechanically roughening such area and applying thereto an aqueous bath consisting essentially of the following proportions of:

G./1. Stannous sulfate 10-40 Sodium bisulfate 100-250 Unsubstituted polyethylene oxide with a molecular weight between 350 and 9000 30-100 Water, suflicient to make 1 liter total.

References Cited in the file of this patent UNITED STATES PATENTS 2,580,773 Heiman Ian. 1, 1952 2,585,902 Gray Feb. 19, 1952 2,624,684 Heiman Jan. 6, 1953 FOREIGN PATENTS 581,034 Great Britain Sept. 27, 1946 

7. IN A PROCESS FOR DEPOSITING TIN ON A SELECTED AREA OF A STEEL SHEET THE STEPS COMPRISING MECHANICALLY ROUGHENING SUCH AREA AND APPLYING THERETO AN AQUEOUS BATH CONSISTING ESSENTIALLY OF THE FOLLOWING PROPORTIONS OF: G./1. STANNOUS SULFATE -------------------------- 10-40 SODIUM BISULFATE ------------------------- 100-250 UNSUBSTITUTED POLYETHYLENE OXIDE HAVING A MOLECULAR WEIGHT OF ABOUT 6000 ---------- 30-100 WATER, SUFFICIENT TO MAKE 1 LITER TOTAL. 